In the Table below, we list some of the main differences (and similarities) between the OSI-409 series (including the ICDR OSI-430) and the new OSI-450. This list is meant to help OSI-409 users that wish to migrate to OSI-450 and need to understand changes between the two. The list is not exhaustive (all the processing chains were revised) and users may observe some differences due to modifications that are not included here. It is recommended to contact the OSI SAF Team whenever you have questions about differences between OSI-409 and OSI-450.

 

OSI-450

OSI-409 series

Comment

Coverage

Global

idem

NH and SH maps in different files

Projection and grid-spacing

EASE2 25km

EASE(1) 12.5km and Polar Stereographic 10km

 

Start Date

Jan 1979

Oct 1978

OSI-450 starts with first ERA-Interim date

Stop Date

Dec 2015

Today minus 31 days

The OSI-409 series is extended operationally with the ICDR OSI-430 . An ICDR is also planned for OSI-450.

Satellite Sensors

SMMR, SSM/I, SSMIS

idem

 

Source of Satellite Data

EUMETSAT CMSAF FCDR R3

Mix of Wentz/RSS, and operational data

 

Intercalibrated and stabilized input satellite data record

Yes (an FCDR)

No (a mix of sources)

 

Use of all available DMSP platforms

Yes

idem

This leads to less data gaps. It is conversely to the NSIDC SIC CDR (that uses one DMSP platform at a time).

Source of NWP Re-analysis Data

ECMWF ERA-Interim

Mix of ECMWF ERA-40, and ECMWF operational analysis

 

Land-mask used

A custom made land-mask starting from a OSTIA 0.05 deg global mask.

A custom made land-mask (source unknown)

The OSI-450 mask is tuned to closely match that of the NSIDC SIC CDR (the NSIDC “SSM/I” 25km Polar Stereographic mask). For OSI-450, on average, this corresponds to setting all EASE2 25x25km grid cells with a fraction of land lower than 30% to water (and can thus be covered with sea ice).

Maximum Ice Extent climatology

A custom monthly climatology based on that of the NSIDC SIC CDR, manually edited, with visual comparison against Climatologies of Canadian and Norwegian Navigational Ice charts.

A custom monthly climatology, based on one from NSIDC, earlier than the NSIDC SIC CDR.

This monthly climatology was fully revisited between OSI-409 and OSI-450.

Processing of SIC on swath data

Yes

idem

The Level2 SICs are later gridded and daily averaged to the output grids.

Brightness Temperature Channels used by the Algorithms

19GHz (V-pol and H-pol), and 37GHz (V-pol and H-pol).

idem

The 4 channels  consistently available from SMMR, SSM/I, and SSMIS.

SIC algorithms used

SICCI2LF

OSISAF “hybrid” algorithm (itself a combination of Bootstrap Freq-Mode, and Bristol algorithms).

The SICCI2LF algorithm was developed in the ESA CCI Sea Ice project.

The NasaTeam algorithm with fixed tie-points is also used to select regions of 100% SIC (to derive dynamic tie-points).

Dynamic tuning of the algorithms tie-points

Yes

idem

The selection of 0% ice tie-point was revised for OSI-450 to be more representative of weather conditions close to the ice edge.

Length of the sliding window for tuning the algorithm tie-points

[-7days;+7days] (15 days)

[-15days;+15days] (31 days)

Shortening the tie-point sliding window allow the 100% ice tie-point to more closely follow emissivity changes at the onset of melting.

Use of NWP data to correct for some of the atmospheric noise, via an RTM.

Yes. Two iterations with double-difference scheme.

idem

OSI-409 used the method of Andersen et al. 2006B. It was simplified for OSI-450 (but performs equally).

NWP variables used for the RTM correction

10m wind speed, Total Columnar Water Vapour, Temp at 2m

idem

 

RTM used

Wentz (1997, JGR)

idem

 

Fields of uncertainties (aka standard errors) in the product file

Yes: Three fields in each file (algorithm uncertainty component, smearing uncertainty component, and total uncertainty - the sum in variance of the two components)

idem

For OSI-450, the algorithm uncertainty methodology was slightly revised to give better results in the mid SIC range. The smearing uncertainty was heavily revised and now shows its max. values along the ice edge (where sharp SIC gradients lead to the larger uncertainty).

Use of an Open Water Filter (aka Weather Filter)

Yes

No

The OWF methodology was developed in the ESA CCI Sea Ice project (and inspired from the Weather Filter of Cavalieri). The tuning of the OWF threshold is dynamic to allow transition across the satellite sensors, and a similar effect across the seasons and hemispheres. A caveat of the OWF (and WF) is that it might remove true sea ice along the ice edge (on average 10% for OSI-450, but can be up to 20-30% in some conditions).

Maps of status flags in the product file

Yes

idem

The format and values are very different between OSI-409 and OSI-450.

Maps of “raw” SIC values in the product file

Yes

No

In OSI-450, an additional netCDF variable holds SIC values before filters are applied, as well as un-constrained SIC values (i.e. < 0% and > 100%). This extra variable is meant for expert users that can benefit from the full Gaussian/Normal distribution of SIC values, instead of just those limited to the [0%-100%] validity range.

Correction of land spill-over effects at swath brightness temperature level

Yes

No

Following Maass and Kaleschke (2010)

Correction of land spill-over effects at daily gridded level

Yes

idem

The methodology was improved for OSI-450. For both OSI-409 and OSI-450, tuning the correction is a trade-off and can lead to removing true ice at the coast. In OSI-450, the “raw” SIC values (before land spill-over correction) can be accessed, to help assess if true coastal sea ice was removed.

Provides interpolated SIC values where satellite data was missing

Yes

idem

The spatio-temporal interpolation was slightly revised for OSI-450. Note that interpolated values are provided to ease display of maps and computation of Sea Ice Extent/Area time-series. They should be used with great care in scientific applications.

Compatibility with the ESA CCI Sea Ice Concentration CDR (v2)

Yes

No

The ESA CCI (AMSR based) SIC CDR uses the same algorithms and processing chains as OSI-450. The same grids, climatologies and land-mask were also used. The file format is the same. It was not known how the SIC values in both CDRs compare at the time this document was written.